Antidepressant drugs are often observed to produce changes in the body mass of patients receiving treatment. We have previously observed that chronic treatment with the monoamine oxidase inhibitor clorgyline alters the body mass of Syrian hamsters. Hamsters treated with this drug failed to increased body mass as did control hamsters. The change in body mass was primarily due to an effect on body lipid content. In our earlier studies we also observed that the failure to increase body mass was not due to a decrease in caloric input since clorgyline-treated hamsters exhibited greater food intake than saline-treated hamsters. Thus, clorgyline produced a condition of negative energy balance compared to control hamsters. To more fully examine the interaction of environment, behavior and metabolism in antidepressant drug-treated hamsters, we have designed a chamber that will allow online, simultaneous measurement of oxygen consumption, motor activity, food intake and body temperature. Ambient temperature can be controlled between 5 and 40 degrees centigrade. Ambient lighting can be programmed to follow numerous regimens including seasonal fluctuations in daylenght. The relationship between parameters which determine energy balance, i.e., energy lost as heat (either during motor activity or during thermoregulation), or energy gained during food intake, can be explored within a circadian perspective.